The present invention relates to a sampler that will remove flue gases from stacks, such as from coal fired power plants, and remove particulate matter from the gases so that the remaining sample can be analyzed accurately for the presence of mercury, in particular, as well as other gases.
Mercury is a significant environmental concern because of its toxicity, persistence, and bio-magnification in the food chain. The chemical form of mercury found in flue gases heavily influences its behavior in control devices and its environmental impact. Therefore, the measurement of mercury in its major chemical and physical forms in flue gases, preferably in real time, is central to efficient design of any mercury control device, to understanding the behavior of mercury in the environment, and to the rational regulation of mercury emissions. Various investigators have presented critical reviews of the importance and difficulty of making reliable measurements of the concentration and speciation of mercury in flue gases. There has been no sampler that insures that a sample of flue gas remains essentially unchanged as it traverses the sampling lines to the analyzer. Particulate matter in the flue gas affects the chemical form of mercury in such gas and its distribution between gaseous and particular components of flue gas. It is now recognized that separating out the particulate matter from the flue gas is a way of insuring that accurate sampling and analysis can take place.
In some instances, filters, cyclones, and cascade impactors will capture and even size fractionate particles in flue gases, and these have been shown to improve the quality of mercury data.
After a relatively short while, these devices reach their capacity for particles and have to be manually cleaned or replaced. Hence they are unsuitable for continuous and/or real time mercury monitoring in a commercial setting. One fundamental limitation of cyclones, impactors, filters, diffusion batteries, electrostatic precipitators and similar devices is that particles accumulate in these devices. Not only does the fresh flue gas pass over the accumulated particles, distorting the measurement of both the particle bound and gaseous mercury species, but the devices do have to be periodically cleaned.
In order to overcome these difficulties. The present invention discloses a variation of a virtual impactor for sampling flue gases, and removing particulate matter from a flow of gases provided to an analyzation instrument, such as a mercury Chemical Environmental Monitor (CEM). Virtual impactors, that have high capacity, are known for sampling particles in the atmosphere, for example, and U.S. Pat. No. 4,670,135 illustrates such a device.
The present invention relates to a sampler mounted within a flue for sampling flue gases and removing particulate material from a flow sample of the gas which is then provided to a an exterior chemical analyzation instrument. The removal of the particles insures that the gaseous chemicals, such as mercury, will not change species or concentration, because influences from the chemical makeup of the particles is removed.
The present invention uses an opposing jet virtual impactor concept, to disengage the particulate matter in the gas stream and accumulate no particulate matter in the sampling device itself. The virtual impactor in the preferred form has been modified to insure that the nozzles and flow passages used are maintained particle free substantially at all times. If it is desired that a cleaning cycle should be used, the cleaning can be done easily and automatically with compressed air and without replacement of components or parts.
The virtual impactor sampler reduces the pressure drop that is associated with many other samplers, and substantially reduces the inadvertent deposition of unwanted particles on the internal surfaces of sample. The virtual impactor uses inlet nozzles aligned with receiver nozzles or tubes into which particles are inertially discharged. While the major gas flow is diverted to a separate flow path, the sampler has two stages of particle separation, in a preferred embodiment shown. The use of a single stage is feasible where there is a low loading of tale sample, and the additional second stage insures the virtual impactor is useable with both high loading and low loading applications and a wide range of flue gas particle size distributions. The sampler is quite compact, and it can be mounted in the flue or stack. The outlet from the sampler for the cleaned or particle free gas is connected to a pump that caused a flow of the gas sample through the analyzation instrument, or mercury CEM that is used in the disclosed embodiment.
The cleaning of the sampler is achieved by utilizing compressed air jets in the exhaust passageways in the sampler body that carry the removed particles from receiver tubes. A high velocity jet flows across receiver passageways that carry the inertially separated particles, insuring that the particles will be discharged immediately and under sufficient pressure to avoid deposition of the particles on passageway surfaces, or on components of the virtual impactor. The jet causes a minor flow of gas through the receiver tubes.
A second form of the invention can be used with valves in the particle exhaust passageways that will close momentarily so that the compressed air will backflush the receiver passageways and if desired the inlet nozzles and passageways of the virtual impactor.
The analyzation instrument can be any desired conventional instrument. The virtual impactor sampler provides a substantially particle free gas for analyzation so that accurate determinations of the chemicals being analyzed is obtained.
In operation, it has been found that the virtual impactor of the present invention accumulates no significant particulate matter and leaves both elemental mercury and mercury chloride unperturbed when sampling flue gases.
The impactor is capable of operating under elevated temperatures, that are commonly found in coal fired power plant stacks or flues, for example, temperatures in the range of 300xc2x0 F. can be handled. The compressed air that is utilized for providing jets for maintaining the virtual impactor free of particles is generally readily available in coal fired power plants, but if necessary, a small compressor can be utilized. A compressed air pressure of between 60 psig and 90 psig is adequate for permitting the sampler to operate at designed conditions.
The sampler will maintain itself free of any particulate matter, as well as fly ash that may be in the flue. The sampler can be a compact unit and thus installed directly into a flue with stack wall connections for the flow of the clean gas sample to the exterior pump and analyzer, and connections for the compressed air pass through the stack wall.